The present invention is related to the field of optical communications and fiber optic networks, and more particularly, provides variable wavelength optical filters for use in tunable laser sources, dense wavelength-division multiplexed systems, and the like.
Variable wavelength devices have a variety of uses in optical systems. One particularly useful variable wavelength device is the tunable laser source. Tunable laser sources often include an external cavity Fabry-Perot laser diode chip in which one end face includes an anti-reflective coating. A wavelength selective reflector is positioned adjacent the coated end face. The wavelength selective reflector is typically a diffractive grating, which may be ruled or holographically made. These reflectors are essentially mirrors with very narrow bandwidths in the wavelength domain. Only a limited portion of the wavelength spectrum is returned to the Fabry-Perot laser diode, and the diode only resonates at the reflected wavelength. By using a wavelength selective reflector which is tunable, the external cavity Fabry-Perot laser diode can be forced to resonate at varying wavelengths, thereby providing a tunable laser source. An exemplary known tunable laser assembly is described in U.S. Pat. No. 5,297,155, the full disclosure of which is incorporated herein by reference.
One way to vary the wavelength of a selective reflector is to rotate a reflective grating so as to change the angle of incidence. This changes the effective period of the grating, and hence the reflected wavelength. Microstepping motors may be used to accurately control the angular position of a grating by directly attaching the grating to the motor output shaft. Although this approach is quite straightforward, resolution and accuracy are limited. High precision microstepping motors may have over 50,000 steps, which still gives an angular resolution of over half a minute. Additionally, angular position errors accumulate during repositioning of a stepper motor. As a result, the wavelength adjustability of the grating is limited, and frequent homing of the microstepper motor is required. Additionally, limiting the bandwidth of a reflective grating to the small ranges desired for dense wavelength-division multiplex systems can be quite difficult. This could lead to mode-hopping problems between signals of adjacent frequencies, so that an etalon is often inserted into the optical path to more cleanly select the desired wavelengths.
It is known to use very different filtering structures for other filtering applications. For example, multi-cavity Fabry-Perot resonators are often built up from multiple thin film coatings to act as bandpass filters. Rather than reflecting a selective bandwidth, such bandpass filters pass a narrow bandwidth distributed about a center wavelength. Manufacturers of thin film bandpass filters generally strive to provide uniform coating thicknesses across the substrate so as to provide a uniform central wavelength. However, specialized bandpass filters having varying coating thicknesses are commercially available from Optical Coating Laboratory, Inc. of Santa Rosa, Calif. under the trademark Selectrabrand(trademark). These variable wavelength filters allow, for example, selection of a specific wavelength within the visual wavelength region by adjusting the filter to an appropriate position when mounting the filter to a detector array package. This can allow a selected portion of the visible spectrum to be scanned electronically.
Co-pending application Ser. No. 09/013,803, filed Jan. 26, 1998 (the full disclosure of which is incorporated herein by reference) describes a structure and method for tuning thin film bandpass filters by varying an angle of incidence of an optical signal directed at the filter. Although this technique allows very fine adjustments of the central bandwidth of the thin film optical filter, polarization dependent loss of the optical signal can be significant when the incident angle is large. Additionally, it may be difficult to control and measure the angle of incidence of light upon the filter when varying the central wavelength after assembly is complete.
In light of the above, it would be desirable to provide improved optical devices, systems, and methods. It would further be desirable to provide improved techniques for filtering and generation of optical signals, particularly for use in dense wavelength division multiplex systems. It would be especially desirable if these improved techniques allowed highly accurate and controllable filtering of optical signals for use in tunable laser sources and other applications within fiber optic networks.
The present invention provides improved optical devices, systems, and methods for controllably varying an optical characteristic of an optical signal, particularly for filtering and generating narrow bandwidth optical signals about a tunable central wavelength. These techniques preferably make use of thin film multi-cavity Fabry-Perot bandpass filters in which the film thicknesses vary so as to vary the center wavelength of the filter""s pass range. To avoid transmission of optical signals through a large area of the optical filter (which would widen the effective pass bandwidth and reduce coupling efficiency by transmitting optical signals through portions of the filter having differing central wavelengths), the invention provides novel fiber-based lenses and lens arrangements. Advantageously, these bandpass filtering techniques can be used in an external cavity laser system to provide a tunable laser source.
In a first aspect, the present invention provides a device for varying a wavelength of an optical signal. The device comprises a first optical fiber disposed along an optical path. An optical filter selectively transmits signals within a pass wavelength range. The pass range of the filter varies from a first pass range at a first region of the filter, to a second pass range at a second region of the filter. A filter moving mechanism is coupled to the filter so that an aligned region of the filter in the optical path moves between the first and second regions. The aligned filter region has an effective pass wavelength range. An electrical component is coupled to the mechanism so as to indicate the effective pass wavelength range.
The central pass frequency of the filter will preferable vary substantially linearly along a filter axis. This allows a threaded filter moving mechanism to accurately position the filter, particularly when the filter is supported by a linear ball slide. Although stepper motors may optionally be used, the electrical component will preferably provide a signal which indicates the absolute position (and hence the absolute bandpass range) of the filter, thereby allowing the use of less expensive motors while providing greater long-term accuracy.
In another aspect, the invention provides a device for varying an optical characteristic of an optical signal. The device comprises an input optical fiber for transmitting the optical signal. The input fiber has an end, and an optical filter is found in an optical path of the optical signal. The filter selectively passes a portion of the optical signal. An output fiber is disposed in the optical path of the passed portion of the optical signal. At least one lens is disposed in the optical path between the input fiber and the output fiber. The at least one lens has a diameter which is not substantially larger than a diameter of the input or output fiber.
In some embodiments, expanded fiber cores adjacent the ends of the input and output fibers will act as lenses to enhance coupling efficiency. When using expanded fiber core lenses, the separation distance between the expanded cores will preferable be less than about one (1) mm, and will ideally be no more than about five hundred (500) xcexcm. In other embodiments, a length of GRIN multi-mode fiber adjacent the end of a single mode input or output fiber may be used as a lens. The length of the GRIN multi-mode fiber will preferably be such that optical signals transmitted from the end of the single mode fiber and through the lens are focused to a waist adjacent the filter.
In yet another aspect, the present invention provides an optical device comprising first and second optical fibers having ends disposed along an optical path. The ends are separated by a distance. A thin film optical element is disposed across the optical path between the ends of the optical fibers. A substrate supports the thin film optical element. The substrate has an opening, the opening receiving the end of the first optical fiber so as to diminish the separation distance.
In yet another aspect, the invention provides an optical fiber lens system comprising a single mode optical fiber having an end. A lens is disposed co-axially with the single mode fiber. The lens comprises a length of multi-mode fiber such that any signals transmitted from the fiber and through the lens are focused toward a waist disposed beyond the lens.
In yet another aspect, the invention provides a variable bandpass filter comprising a substrate having a first major surface and a plurality of coatings disposed on the first surface. The coatings selectively transmit light signals through the substrate within a pass wavelength range having a central wavelength. At least one of the coating thicknesses varies along a line between a first region and a second region.
In yet another aspect, the invention provides a tunable laser source comprising a laser diode and a reflector disposed along an optical path from the laser diode. The reflector is positioned so as to define an external laser cavity between the laser diode and the reflector. A bandpass filter is disposed along this external cavity. The filter has a pass range. A filter varying mechanism is coupled to the filter to vary the pass range of the filter from a first pass range to a second pass range.